All regulatory-code-compliant, utility-grid-interactive power converters must operate according to grid-connect standards for a given jurisdiction. In the United States, the governing standard is IEEE Standard for Interconnecting Distributed Resources with Electric Power Systems, commonly known as IEEE 1547. In short, this standard specifies that distributed resources, such a solar photovoltaic power converters, must provide a minimum power quality, must not operate outside of set utility voltage and frequency limits and must detect an islanding condition and automatically cease power production.
Islanding or run-on is an abnormal operating condition where a grid-interactive power converter continues to supply power to grid-connected loads following the loss of the utility grid source. In simple terms, all grid-interactive power converters are current sources, whereas the grid source, typically the electric utility grid, is a voltage source. The power converters, the grid source and grid-connected loads are connected at a point commonly referred to as the point of common coupling (PCC). All anti-islanding detection algorithms use passive and active methods. The passive method involves monitoring the voltage and frequency at the PCC and checking for voltages or frequencies outside of specified limits. The passive method alone will not detect an island under balanced conditions where available power out of the converter matches the power into grid-connected loads. All active methods assume a worst-case balanced condition and periodically and purposefully attempt to disturb a balanced condition to force the voltage or frequency at the PCC past passive detection trip points. Almost all prior art disturbance methods are integrated with the power converter control systems and work by periodically jogging the amplitude or phase of the power converter sinusoidal current references to force a detectable change at the PCC. If the grid voltage source is present, purposefully jogging or distorting the power converter output current will not affect the voltage or frequency at the PCC. If however, the grid voltage source is lost, and therefore is not controlling the voltage at the PCC, then sufficient changes in power converter current can result in detectable changes in voltage or frequency across the grid-connected loads. IEEE 1547 requires that an island condition be detected and that the power converter either be shutdown or disconnected from the PCC within two seconds after formation of an island. As such, under all conditions where the power converter is sourcing power at the PCC, the power converter current is typically shifted in phase or amplitude for at least one cycle every second to “test” for an island.
Prior art approaches to anti-islanding protection have a number of basic limitations. (i) Distortion—Ideally, the power converter would source perfect, low-harmonic-distortion sinewave currents into the grid voltage source at the PCC. Prior art methods rely on purposefully distorting the converter output current, typically for at least one cycle every second, as part of the island detection algorithm. (ii) Interaction—When multiple power converters are connected at the PCC, interactions between individual power converter anti-islanding algorithms may cause nuisance tripping or in the other extreme may cause an island to go undetected. (iii) Non Autonomous Operation—Almost all prior art methods use the power converter control as an integral part of the anti-islanding algorithm. (iv) Absolute Trip Levels—The magnitude of grid disturbance required with prior art methods must be great enough force the grid past absolute under/over, voltage/frequency trip points. (v) Cost—In prior art systems with a large number of distributed DC-to-AC power converters, each inverter is required to have an integrated anti-islanding protection system.